This invention relates to hydraulic actuation systems and in particular hydraulic actuation systems for automated transmission systems.
In automated transmission systems of, for example, the type disclosed in WO97/05410 or WO97/40300, whose content is expressly incorporated in the disclosure content of the present application, fluid pressure actuators are used to control actuation of a clutch actuator mechanism and/or a gear engaging mechanism. In accordance with WO97/05410, separate control valves are used to control the clutch actuator mechanism and the gear engaging mechanism.
WO97/40300 discloses a hydraulic actuation system in which a main control valve controls both the clutch actuation mechanism and, together with secondary valves, shift and select actuators of a gear engaging mechanism. The use of a single main control valve in this manner reduces the number of components, providing savings in the overall size and cost of the system. The design of the master control valve is however significantly more complicated, which reduces the cost savings.
Hitherto, hydraulic pressure for clutch actuation and for shifting gears has been supplied from a gas accumulator which is charged by means of an electrically driven pump. The accumulator provides an immediate supply of pressurised fluid, to disengage the clutch, upon initiation of a gear change. The use of an accumulator also enables the use of a smaller pump.
Typically the pressure required to operate the clutch will be of the order of 30 bar while the pressure required for gear shifting may be as high as 60 bar, but will generally be about 20 bar. However, in order to store sufficient volume for clutch actuation and shifting of gears, pressure in the accumulator must be as high as 60 bar. The pump has consequently to work at 60 bars to charge the accumulator in order to store the required volume of fluid.
Moreover, it is desirable during the shifting of gears to vary the force applied to the shift actuator, when, for example, engaging the syncromesh. This has been achieved hitherto by using a pressure transducer to measure the pressure of fluid supplied to the shift actuator and modulating the flow of fluid into the system using separate proportional flow control valves to maintain the correct pressure.
In accordance with co-pending UK Application (our reference P1405/KTM-EM113) filed on even date herewith, a hydraulic actuation system for an automated transmission system comprises; a hydraulic clutch actuator for controlling engagement of a clutch; a gear engagement actuator for controlling engagement of a gear; an accumulator; an electric motor driven constant displacement pump, an output of the motor driven pump being connected to the accumulator for charging thereof via a non-return valve; a main control valve for selectively connecting the accumulator to the hydraulic clutch actuator and/or the gear engagement actuator; a pressure transducer arranged to measure pressure of fluid in the accumulator; and a control means, said control means controlling energisation of the main control valve to effect a gear change.
Pressure of fluid in the system described above and in particular that delivered to the gear engagement actuator during a gear change, may be controlled by switching the pump on and off at predetermined times. In order to achieve accurate control of pressure during a gear change, it may be necessary to switch the pump on and off several times during the change. Moreover, as the pump will take time to run up to full speed and come to a complete stop, pressure control in this manner is complex.
According to one aspect of the present invention provides, a method of controlling a hydraulic actuation system for an automated transmission system having;
a hydraulic clutch actuator for controlling engagement of a clutch;
a gear engagement actuator for controlling engagement of a gear;
an accumulator;
an electric motor driven constant displacement pump, an output of the motor driven pump being connected to the accumulator for charging thereof via a non-return valve,
a main control valve for selectively connecting the accumulator to the hydraulic clutch actuator and/or the gear engagement actuator;
a pressure transducer arranged to measure pressure of fluid in the accumulator; and
a control means, said control means controlling energisation of the main control valve to effect a gear change;
said method comprising;
upon initiation of a gear change opening the clutch actuator to the accumulator to effect disengagement of the clutch;
isolating the clutch actuator from the accumulator in order to clamp the clutch in a disengaged position at a point where the clutch is disengaged to an extent that it does not transmit torque but has not reached the extent of movement of the clutch actuator in the disengagement direction;
switching on the pump motor to provide a predetermined accumulator pressure at a predetermined stage of the gear change; and
reconnecting the clutch actuator to the accumulator to maintain the predetermined pressure or to change the pressure from the predetermined pressure in controlled manner.
In this manner the reopening of the clutch actuator to the accumulator, so that it will move further towards the fully disengaged position of the clutch, will divert fluid to the clutch actuator, countering the increase in pressure due to operation of the pump. This effect will be substantially instantaneous, permitting fine control and reducing the frequency at which the pump must be switched on and off.
A combination of switching the pump on and off at predetermined times and reconnecting the clutch actuator to the accumulator may consequently be used to control pressure in the system. Furthermore, the rate at which fluid is delivered to the clutch actuator may be controlled by means of the main control valve, so that it matches, is higher or is lower than that of the pump, so that the accumulator pressure may be held constant, increase at a lower rate than normal pump rate or decrease at a controlled rate.
In accordance with a preferred embodiment of the invention a spring accumulator is used. The spring accumulator will also provide consistent compliance in the system further enabling accurate pressure control.
The pressure transducer is preferably used in a closed loop feedback system to control accumulator pressure.